It is a common practice to give supplemental iron to all pregnant women world-wide. The dose can vary from 60 mg/d to as much as 240 mg/d. When therapeutic doses of iron are prescribed, it is rare to also prescribe supplemental zinc. Studies in experimental animals and humans show that supplemental iron interferes with zinc absorption. In a recent longitudinal study of zinc absorption during pregnancy done by us, four of the 14 women studied were prescribed supplemental iron by their physicians. Although zinc absorption increased during pregnancy reaching a significant change during lactation, this adjustment was not observed in the four women taking supplemental iron. The need for absorbed zinc increases by 0.7 mg/d during pregnancy and 1.4 mg/d during early lactation. Our study showed that this need is met by upregulating zinc absorption if supplemental iron is not given. Administration of supplemental iron may impair this change in zinc use during pregnancy and lactation. The purpose of this proposed study, therefore, is to define the effect of supplemental iron on the metabolic adjustments in zinc utilization in healthy women during pregnancy and lactation. We hypothesize that a daily supplement of 100 mg iron inhibits zinc absorption and reduces the net uptake of zinc from the gastro-intestinal tract during late pregnancy and early lactation when the additional need for zinc is the greatest. Two groups of 15 women each will be recruited in early pregnancy and will be studied at 12-14, 32-34 weeks of gestation, and at 4-6 weeks postpartum. One group will receive 100 mg supplemental iron daily; the other group will be unsupplemented. All women will be given a prenatal vitamin supplement plus 8 mg iron and 5 mg zinc to assure a stable basal intake of the micronutrients throughout the study. Using stable isotopes, iron and zinc absorption and zinc kinetics will be measured. Biochemical measures of iron and zinc status will also be determined. All women will reside in a metabolic unit for 6 days during each test. Results from this comprehensive, longitudinal study will provide new data on the effects of iron supplementation on zinc utilization during pregnancy and lactation.